This is a great experiment. I just added some money to the card. I wonder to prevent abuse, if perhaps you should use two cards , one for people to give to (A)

This is a great experiment. I just added some money to the card. I wonder to prevent abuse, if perhaps you should use two cards , one for people to give to (A) and the other to use at starbucks (B). You can transfer from (A) to (B), when the balance of (B) goes below a certain amount say $15 so that one person can't abuse the card by taking away all the existing balance.